1. Field of the Invention
The present invention relates to a toner for use in an electrophotographic image forming apparatus. In addition, the present invention also relates to a toner manufacturing method; an image forming apparatus and a process cartridge using the toner.
2. Discussion of the Background
When a toner image formed by an electrophotographic image forming method is fixed, contact heat fixing methods such as a heat roller fixing method are typically used. A fixing device used for the heat roller fixing methods includes a heating roller and a pressing roller. A toner image formed on a recording material is fixed by passing through a nip formed between the heating roller and the pressing roller.
In contact heat fixing methods, because the surface of a heating member (such as the heating roller) directly contacts the toner image, a part of the toner is adhered to the heating member and then re-transferred to the following sheet of the recording material, resulting in occurrence of the offset problem.
In attempting to prevent occurrence of the offset problem, a technique in which a fixing oil such as a silicon oil is applied or penetrated to the heating roller and the pressing roller, is proposed. However, in order to downsize the device and lower the operation costs, oilless fixing devices without a fixing oil applying system or fixing devices applying a small amount of oil have been typically used. For these fixing devices, toners including a release agent serving as an offset inhibitor are used.
On the other hand, in heat fixing methods, the heating temperature is preferably as low as possible from the viewpoint of energy saving. However, a toner binder resin having too low thermal properties (such as low-temperature fixability) has poor thermostable preservability and causes a toner blocking problem. Polyester resins are preferably used as toner binder resins because of having both low-temperature fixability and thermostable preservability. This is because polyester resins have a relatively low viscosity and a high elasticity compared to vinyl copolymer resins.
However, when a toner including a large amount of release agent is manufactured by a conventional pulverization method, a major portion of release agent tends to exist on the surface of the resultant toner and causes a toner filming problem and a blocking problem. On the other hand, polymerization methods such as the suspension polymerization method in which reactive monomers are polymerized in an aqueous medium, and the emulsion aggregation method in which emulsion polymerization particles are aggregated in an aqueous medium, are known as toner manufacturing methods. Toners manufactured by these polymerization methods can include a release agent in an amount much greater than in the case of a pulverization method. Japanese Patent No. 3195362 discloses a suspension polymerization method in which the toner structure is controlled by further polymerizing monomers with granulated (polymerized) mother toner particles. Published unexamined Japanese Patent Application No. 2002-116574 (hereinafter referred to as JP-A) discloses an emulsion aggregation method in which the toner structure is controlled by further adding emulsion polymerization particles to granulated (polymerized) mother toner particles. However, resins used for the suspension polymerization method and the emulsion aggregation method are vinyl resins because these reactions are performed in an aqueous medium. It is difficult to use polyester resins because the polyester is polymerized at a high temperature of about 200° C.
In emulsion aggregation methods, a metal salt is typically used as an aggregation agent. Therefore, the metal salt is incorporated in the resultant toner particles, but it is difficult to remove all of the incorporated metal salt in a washing process. Thereby, the resultant toner is largely affected by environmental conditions such as humidity.
As a toner manufacturing method using polyester resins, a solution suspension method in which a polymerized polymer, which is dissolved in an organic solvent to prepare a toner constituent solution, is emulsified to be granulated in an aqueous medium, is known. In this method, the molecular weight of the toner equals that of the binder resin. In order to control the thermal properties of the toner, a combination of a high-molecular-weight resin and a low-molecular-weight resin are typically used. However, when a high-molecular-weight resin is dissolved in a solvent, the viscosity of the solution is so large that the solution has poor granulation ability. In order to avoid such a problem, a method is used in which the molecular weight of the low-molecular-weight resin is set to be relatively high, instead of using a large amount of high-molecular-weight resin. In this case, the resultant toner has poor low-temperature fixability.
In attempting to solve such a problem caused by using high-molecular-weight resins, a toner manufacturing method using a modified polyester having a reactive group is proposed. In this method, the modified polyester having a reactive group is elongated or cross-linked after granulation of toner particles while controlling the molecular weight (i.e., thermal property.) However, the method has a drawback in that the toner structure cannot be sufficiently controlled and thereby colorants and release agents tend to exist on the surface of the toner.
JP-As 2000-292973 and 2000-292978 have disclosed toners having a particulate resin on the surface of the toner particles to improve hot offset resistance. However, the low-temperature fixability of the toner is insufficient. In addition, salting out fine particles on core particles takes a long time resulting in increase of the manufacturing cost.